Changing Gears in Halfbike3

Hi, I have been riding for 6 months now, I quite good at it :-); however I cant figure out how to get the most of my gear box. I am just using 1 gear. (3rd). Can anyone give men some tips?

I have a Halfbike 3 with 4 gears. Usually on flat ground I start with 3rd gear and after that I switch to 4th. On hilly terrain I use 2nd and in rear situations 1st.